Load dispersal through fill above box underpass

[lee_williams1]

Good afternoon,

I am relatively new to SBD and I am currently using SBD 2022.

I am assessing a box underpass to CS 454/CS 455 to try to improve the capacity of the structure. The proposed method of analysis of the reinforced concrete box underpass is to use 3D elastic FEA. I have created the model and I am using the influence surface generator and load optimiser to determine worst-case loading scenarios for ALL Model 1 loads (I will be applying HB, SV, and SOV models too).

However, the box underpass has fill above the structure which will help disperse the load which will be important in maximizing the assessed capacity of the structure. My question is: what is the best method of representing the fill above the structure to allow for load dispersal from live load?

I have considered increasing the wheel contact area to represent dispersal, inserting a mesh over the structure to represent soil properties, and factoring the applied loading. However, I am also keen to use the influence surface generator for the efficiency benefit it brings so I don't think I can factor the applied load and still use the influence surface generator.

My model currently has the carriageway directly onto the FE elements as shown below:

I am hoping the advice will be applicable in the application of not only ALL Model 1 loading, but also for the application of HB,SV, and SOV loading too.

Thanks in advance,

Lee

[dave_geeves · Accepted solution]

Hi,

A great question and thanks for describing your issue in such great detail.

I have very often tried to model the dispersion  of traffic load through a level of fill onto the structure by using similar methods that you have described. However, I have found that this only makes a significant difference if it is possible to distribute the load beyond the bounds of the loaded deck - as I suspect in your case.

It is important to realise that in ASBD, all vehicle loads (wheel loads, UDL, axle Loads, HA, SV and SOV) are applied as a series of point loads on nodes in the deck.  If a wheel load is applied in the centre of a rectangular finite element in the deck it will get distributed to the four nodal loads to the nodes in the element.  So, essentially there is some distribution happening here by default.  However, I think in your case you are more interested in looking at situations where some of the load is to be distributed into the soil/fill outside of the structure.  Currently the software does not cater for this in the load optimisation or in fact any placing of vehicle loads.

If you are going to create and additional deck of elements at the surface of the road and attach this to the culvert structure to distribute the loads then it is important to ensure that this deck extends beyond the structure so that load is picked up outside the bounds of the structure. I have found that the connection between the road surface and the structure is best achieved with a latticework of vertical finite elements, but the thickness and properties need to be defined by trial and error. The nodes of the fictitious fill outside of the structure will need to be supported to represent the resistance of the ground/fill.

So, to summarise, I don't have a definitive answer, but in general, most things can be modelled with care and certain assumptions, but, it is important to realise that the more detailed the model the more difficult it is to check.

I hope this helped, and if it does, please mark my post as a solution so that others may benefit.

Kind regards,

Dave Geeves

[lee_williams1]

Thanks for your response Dave,

Yes - the aim is to disperse load beyond the bridge deck since it is a small span structure, the number of axles applied at any point is limited.

I have accepted the solution however, I will try a few scenarios out in the model, and any findings I make I will post back into this topic.

Thanks again, Lee